Scale-dependent linkages between nitrate isotopes and denitrification in surface soils: implications for isotope measurements and models
- 461 Downloads
Natural abundance nitrate (NO3 −) isotopes represent a powerful tool for assessing denitrification, yet the scale and context dependence of relationships between isotopes and denitrification have received little attention, especially in surface soils. We measured the NO3 − isotope compositions in soil extractions and lysimeter water from a semi-arid meadow and lawn during snowmelt, along with the denitrification potential, bulk O2, and a proxy for anaerobic microsites. Denitrification potential varied by three orders of magnitude and the slope of δ18O/δ15N in soil-extracted NO3 − from all samples measured 1.04 ± 0.12 (R 2 = 0.64, p < 0.0001), consistent with fractionation from denitrification. However, δ15N of extracted NO3 − was often lower than bulk soil δ15N (by up to 24 ‰), indicative of fractionation during nitrification that was partially overprinted by denitrification. Mean NO3 − isotopes in lysimeter water differed from soil extractions by up to 19 ‰ in δ18O and 12 ‰ in δ15N, indicating distinct biogeochemical processing in relatively mobile water versus soil microsites. This implies that NO3 − isotopes in streams, which are predominantly fed by mobile water, do not fully reflect terrestrial soil N cycling. Relationships between potential denitrification and δ15N of extracted NO3 − showed a strong threshold effect culminating in a null relationship at high denitrification rates. Our observations of (1) competing fractionation from nitrification and denitrification in redox-heterogeneous surface soils, (2) large NO3 − isotopic differences between relatively immobile and mobile water pools, (3) and the spatial dependence of δ18O/δ15N relationships suggest caution in using NO3 − isotopes to infer site or watershed-scale patterns in denitrification.
KeywordsIsotope mass balance model Mobile water Nitrification Redox Snowmelt
The manuscript was greatly improved by critical feedback from Jason Kaye and two anonymous reviewers. We gratefully acknowledge field and lab assistance from Simone Jackson, Jillian Turner, Dave Eiriksson, Kendalynn Morris, and contributions from Suvankar Chakraborty, Gabe Bowen, and Jim Ehleringer in implementing the denitrifier method at SIRFER. This research was supported by NSF EPSCoR grant IIA 1208732 awarded to Utah State University, as part of the State of Utah Research Infrastructure Improvement Award, and by NSF grant DBI-1337947. Any opinions, findings, and conclusions or recommendations expressed are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
Author contribution statement
S.J.H. designed the study, S.R.W. and D.R.B. contributed to sample analysis and interpretation, and S.J.H. wrote the paper with contributions from S.R.W. and D.R.B.
- Billy C, Billen G, Sebilo M, Birgand F, Tournebize J (2010) Nitrogen isotopic composition of leached nitrate and soil organic matter as an indicator of denitrification in a sloping drained agricultural plot and adjacent uncultivated riparian buffer strips. Soil Biol Biochem 42:108–117. doi: 10.1016/j.soilbio.2009.09.026 CrossRefGoogle Scholar
- Brooks PD, Williams MW (1999) Snowpack controls on nitrogen cycling and export in seasonally snow-covered catchments. Hydrol Process 13:2177–2190. doi: 10.1002/(SICI)1099-1085(199910)13:14/15<2177:AID-HYP850>3.0.CO;2-V CrossRefGoogle Scholar
- Castellano MJ, Lewis DB, Kaye JP (2013) Response of soil nitrogen retention to the interactive effects of soil texture, hydrology, and organic matter. J Geophys Res Biogeosci 118:280–290. doi: 10.1002/jgrg.20015
- Cohen MJ, Heffernan JB, Albertin A, Martin JB (2012) Inference of riverine nitrogen processing from longitudinal and diel variation in dual nitrate isotopes. J Geophys Res Biogeosci 117:G01021. doi: 10.1029/2011JG001715
- Craine JM, Elmore AJ, Wang L, Augusto L, Baisden WT, Brookshire ENJ, Cramer MD, Hasselquist NJ, Hobbie EA, Kahmen A, Koba K, Kranabetter JM, Mack MC, Marin-Spiotta E, Mayor JR, McLauchlan KK, Michelsen A, Nardoto GB, Oliveira RS, Perakis SS, Peri PL, Quesada CA, Richter A, Schipper LA, Stevenson BA, Turner BL, Viani RAG, Wanek W, Zeller B (2015) Convergence of soil nitrogen isotopes across global climate gradients. Sci Rep 5:8280. doi: 10.1038/srep08280 CrossRefPubMedPubMedCentralGoogle Scholar
- Fang Y, Koba K, Makabe A, Takahashi C, Zhu W, Hayashi T, Hokari AA, Urakawa R, Bai E, Houlton BZ, Xi D, Zhang S, Matsushita K, Tu Y, Liu D, Zhu F, Wang Z, Zhou G, Chen D, Makita T, Toda H, Liu X, Chen Q, Zhang D, Li Y, Yoh M (2015) Microbial denitrification dominates nitrate losses from forest ecosystems. Proc Natl Acad Sci USA 112:1470–1474. doi: 10.1073/pnas.1416776112
- Groffman PM, Holland EA, Myrold DD, Robertson GP, Zou X (1999) Denitrification. In: Robertson GP, Bledsoe CS, Coleman DC, Sollins P (eds) Standard soil methods for long-term ecological research. Oxford University Press, New York, pp 272–290Google Scholar
- Houlton BZ, Bai E (2009) Imprint of denitrifying bacteria on the global terrestrial biosphere. Proc Natl Acad Sci USA 106:21713–21716. doi: 10.1073/pnas.0912111106
- Houlton BZ, Sigman DM, Hedin LO (2006) Isotopic evidence for large gaseous nitrogen losses from tropical rainforests. Proc Natl Acad Sci USA 103:8745–8750. doi: 10.1073/pnas.0510185103
- Kendall C, Elliott EM, Wankel SD (2007) Tracing anthropogenic inputs of nitrogen to ecosystems. In: Michener R, Lajtha K (eds) Stable isotopes in ecology and environmental science. Blackwell, London, pp 375–449Google Scholar
- Mariotti A, Germon JC, Hubert P, Kaiser P, Letolle R, Tardieux A, Tardieux P (1981) Experimental determination of nitrogen kinetic isotope fractionation: some principles; illustration for the denitrification and nitrification processes. Plant Soil 62:413–430. doi: 10.1007/BF02374138 CrossRefGoogle Scholar
- Petersen DG, Blazewicz SJ, Firestone M, Herman DJ, Turetsky M, Waldrop M (2012) Abundance of microbial genes associated with nitrogen cycling as indices of biogeochemical process rates across a vegetation gradient in Alaska. Environ Microbiol 14:993–1008. doi: 10.1111/j.1462-2920.2011.02679.x CrossRefPubMedGoogle Scholar
- Sonderegger D (2012) SiZer: Significant Zero Crossings. R package version 0.1-4. http://CRAN.R-project.org/package=SiZer. Accessed 1 Oct 2015
- Zak DR, Groffman PM, Pregitzer KS, Christensen S, Tiedje JM (1990) The vernal dam: plant–microbe competition for nitrogen in northern hardwood forests. Ecology 71:651–656. doi: 10.2307/1940319